First at all, why did you knew that it was rolling off? Did you used measurement equipment to check it? Or did you use any software tester like rightmark and a soundcard?
Because I have, and mine outputs straight as a line. Please, do that before making assumptions based on subjective impressions. Post your picture from rightmark too so we can aim to diagnose your problem.

Going from metal films to carbon composition is not going to make any objective difference.

Can you post rightmark measurements of your build? I wanna see how they compare to dedicated FFT software. What soundcard?

So now its subjectively flat? Because to such a degree of treble loss you describe it should have been a very serious problem. Most likely oscillation. Do you have long interconnects on input/output? What type of wire construction, shielding?

Yes, it is now subjectively flat. Sounds about the same in response as the NAD C160 preamp that it replaced. The interconnects I am using are 1 meter Belkin. The test build is fairly amateur. I've used what I had to make it work before I rebuild it into something nice. I did have some small signal shielded coax for the input and output. I also used some solid core copper telephone wire to finish connecting the jacks and volume control to the coax. It isn't pretty but I just wanted to test the circuit before I ordered any more parts.

Another way that I could describe what it sounded like when it had the problem is this. Imagine you come home and notice that almost all the treble was gone when you turned your stereo on. You have a pair of two way speakers that cross over at about 1500 Hz. Your son got happy with your stereo while you were out and killed both of the tweeters. It was about that bad.

Sure I have made my measurements. Give it an eye and you will see some odd differences.

My soundcard is an EMU0404 USB wich has been changed the dc blocking capacitors for some pannasonic FM's. I made a loop with two homemade TR->RCA cables from its output to its input. Then I placed my DCB1 with its LDR volume control inside the loop.

I will upload here my measurements WITH the buffer. In my next post are my measurements without the buffer,

The software is Rightmark Audio Analyzer. And it is a very easy and effective software to use, I encourage to try it. You have to connect the output of a soundcar to the input o fthe same or another sound card in the same computer. Run the software, is free, and put between input and output as many audio gear as you want to measure in chain.
Be careful with doing things like hooking up a power amplifier. IT WILL MAKE FIRE.
Preamps, buffers, and in general all line level devices use to be safe.

Regards,
Regi

__________________
diyAudio, doing it as big as you can, JUST BECAUSE WE CAN!

I see much harmonic noise. Does it still have it when you loop the card alone? You may need to ground lift. Mine FFTs you have seen recently. This pdf one is Grufti's. He had posted long before. My system shows the floor down to -140dB to the right with the DCB1 in, so the needle on Grufti's shows also its root along with some interference in the EMU to the far right which is in its loop with those software settings.

Your son got happy with your stereo while you were out and killed both of the tweeters. It was about that bad.

Usually it's me who kills tweeters or even midrange horns here, not my son :-). But up to now this is valid only for the (poor) PA-system I have to play with our band. My hifi-system never got a dead tweeter (touch wood), only a dead woofer (rubber ring worn out). But I understand what you mean. You listen, clean your ears, listen again, discover the dead parts. And then search for someone who killed them.

I managed a bit better than that with some parameters tweaking and less level, but that's the vicinity for the Tracker. Same Y axis as above with DCB1. Basically it buffers the loop and gets even better for THD even at higher level.